OSRI / OSFI — One Shot Rising and Falling with Input

One Shot Rising with Input (OSRI). Structured Text one-shot function block that sets OutputBit for exactly one scan when InputBit goes from 0 to 1. Uses FBD_ONESHOT; the ST equivalent of ladder OSR.

One Shot Falling with Input (OSFI). Structured Text one-shot function block that sets OutputBit for exactly one scan when InputBit goes from 1 to 0. Uses FBD_ONESHOT; the ST equivalent of ladder OSF.

An edge is the single scan a signal changes — not the state itself. A one-shot turns "the button is held down" into "the button was pressed just now," so counters count once per press instead of once per scan.

Operands

Name	Type
oneShot	FBD_ONESHOT

Each edge you watch needs its own FBD_ONESHOT tag — the structure remembers the previous scan's InputBit internally.

FBD_ONESHOT Tag Members

Member	Type	Meaning
.EnableIn	BOOL	Forced to 1 each scan by the instruction call — setting it 0 has no effect
.InputBit	BOOL	Input the one-shot watches — assign it before calling the instruction each scan
.EnableOut	BOOL	1 while the instruction is executing normally
.OutputBit	BOOL	1 for exactly one scan after the watched InputBit edge — rising for OSRI, falling for OSFI

How They Work

Each scan, the instruction compares InputBit now against what it was on the previous scan:

• OSRI — InputBit goes 0 → 1: OutputBit is 1 for that one scan. While InputBit stays 1 (or stays 0), OutputBit is 0.
• OSFI — InputBit goes 1 → 0: OutputBit is 1 for that one scan. A falling edge is not the same as "the input is off" — OSFI pulses only on the scan the input turns off.

Call the instruction every scan and assign .InputBit before the call — the edge is detected at the moment of the call:

EdgeTag.InputBit := In_Signal;
OSRI(EdgeTag);
Out_Pulse := EdgeTag.OutputBit;

Neither instruction fires on the first scan for a signal that was already settled at startup. Prescan arms the internal edge memory — OSRI treats an input that is already 1 as old news, and OSFI has no stored "1" for a falling edge to measure against.

InputBit   _____|‾‾‾‾‾‾‾‾|________|‾‾‾‾‾‾|___
OSRI       _____|‾|_______________|‾|________
OSFI       _____________|‾|______________|‾|_

Example — Count Button Presses

OSRI fires once per press no matter how long the button is held:

One_Press.InputBit := In_Button;
OSRI(One_Press);

IF One_Press.OutputBit THEN
    Val_Count := Val_Count + 1;
END_IF;

Example — Act When a Machine Stops

OSFI catches the moment In_Running drops, latching a done flag exactly once per cycle:

One_Stopped.InputBit := In_Running;
OSFI(One_Stopped);

IF One_Stopped.OutputBit THEN
    Out_CycleDone := 1;
END_IF;

Common Mistakes

• Reading .OutputBit on a line above the instruction call — you get the previous scan's pulse, one scan late. Assign .InputBit, call the instruction, then read .OutputBit, in that order.
• Reusing one FBD_ONESHOT tag for two different signals (or calling the instruction twice on the same tag in one scan) — the second use consumes the edge memory. One tag per watched edge.
• Confusing a falling edge with an inverted input — NOT In_Signal is 1 the whole time the signal is off; OSFI pulses for a single scan when it turns off.
• Expecting a pulse on the first scan because the input starts at 1 — prescan arms the edge memory, so a settled input is not an edge. OSFI needs InputBit observed at 1 during a scan before the drop to 0 counts.

Related

• OSR / OSF — the ladder-logic equivalents
• ONS — ladder rising-edge one-shot that conditions its own rung
• CTUD — the ST counter, which has the same edge-armed prescan behavior on CUEnable/CDEnable